2. Pneumatic Starter
A. The pneumatic starter (Fig. 2) is a lightweight turbine-type air motor which converts the kinetic energy of compressed air into starting torque sufficient to accelerate the engine to starting speed. Low pressure air and electrical power are required for starter operation. The starter will continue to assist the engine until electrical power is removed.
B. The starter consists of a scroll assembly, turbine wheel, reduction gear assembly, engaging mechanism and an output shaft. The starter is fitted with a start valve to control the inlet airflow. When the valve is open, it admits air to the inlet connection on the starter scroll assembly; the air then passes through the starter vanes of the scroll assembly and is directed radially inward through the turbine wheel imparting high-speed rotation. Exhaust air from the turbine wheel then passes through an air outlet screen.
C. The reduction gear train translates the high speed, low torque of the turbine wheel into low speed, high torque. This output is transmitted through a pawl and ratchet engagement mechanism to the output shaft. From the starter output shaft the cranking torque is transmitted to the N2 compressor by way of the accessory drive gears. A clutch mechanism provides engagement of the reduction gear train with the output shaft for engine starting; when the speed of the output shaft exceeds the speed of the internal gear hub, the clutch mechanism overruns, thus providing automatic disengagement. The pneumatic starter is mounted on the accessory drive gear case beneath the engine.
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5A1 Pneumatic Starter
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3. Starter Valve (Fig. 3)
A. The starter valve controls airflow to the pneumatic starter during engine starting. The valve is pneumatically operated and solenoid-controlled. It consists essentially of two assemblies; a valve body assembly with lever-operated butterfly valve, and a piston type pneumatic actuator assembly. The valve is located in the pneumatic duct to the starter. It is attached to the duct and starter inlet flange by clamps (Fig. 3).
B. Valve P/N 898172-1-1 (AiResearch)
(1)
Valve inlet pressure is directed tttttttttthrough a pressure port, filter, and orifice, into a ball-type solenoid-actuated switcher and through the pneumatic switcher to chamber B. With solenoid de-energized, chamber A is vented and inlet pressure in chamber B keeps the butterfly closed. With solenoid energized, the ball closes the vent, and inlet pressure is ported to chamber A and to the actuator of the pneumatic switcher. The pneumatic switcher closes off inlet pressure to chamber B and connects that chamber to sensing selector.
(2)
As chamber B vents through the bleed orifice, pressure differential across the actuator piston moves the butterfly toward open position. At inlet pressures lower than set valve (36 +5 psig), sensing selector remains closed and keeps the butterfly in fully open position. When inlet pressures equal or exceed set valve, bleedoff reference regulator opens to maintain a constant reference pressure in chamber A, and the sensing selector opens to direct downstream pressure to chamber B. This modulates the butterfly position to maintain the desired downstream pressure.
(3)
When solenoid is de-energized, pneumatic switcher and chamber A are quickly vented and upstream pressure is ported to chamber B or closing side of the actuator, causing rapid closing of the butterfly. As downstream pressure drops, sensing selector closes, chamber B pressure rises, assisting the torsion spring in closing the valve completely.
(4)
An additional pneumatic bleed located on the upstream side of the butterfly vane directs hot air onto the solenoid switcher and pneumatic switcher housings to prevent icing and to reduce moisture during cold weather operation.
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
3. Starter Valve (Fig. 3)
A. The starter valve controls airflow to the pneumatic starter during engine starting. The valve is pneumatically operated and solenoid-controlled. It consists essentially of two assemblies; a valve body assembly with lever-operated butterfly valve, and a piston type pneumatic actuator assembly. The valve is located in the pneumatic duct to the starter. It is attached to the duct and starter inlet flange by clamps (Fig. 3).
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